1. Field of the Invention
This invention is related to a common ground control circuit for an irrigation system and, more specifically, to a common ground control switch for an irrigation system which controls the operation of electrically operated irrigation equipment associated therewith based upon the level of moisture in the ground.
2. Description of Related Art
Irrigation systems may be classified by the source of water and the manner in which water is distributed to the land. In terms of distributing irrigation water, there are four basic methods: (1) flood irrigation, (2) furrow irrigation, (3) subirrigation system and (4) overhead irrigation system. An overhead irrigation system artifically distributes water over the ground. Such systems mimic natural rain conditions by spraying the water over fields, lawns or other designated areas of land. One such overhead irrigation system is commonly referred to as a sprinkler system. Sprinkler systems, which may be of either the portable, semi-permanent, or permanent variety depending on whether the lateral pipeline (including the sprinkler), main pipeline, and pumping plant are moveable or fixed, typically operate by the piping of water under pressure through a sprinkler or perforations or nozzles in a pipeline, thus forming a spray. For land which surrounds private residences, the use of permanent-type overhead irrigation systems having fixed sprinklers are common. As previously mentioned, however, such sprinkler systems merely mimic natural rain conditions and, as will be described below, will often produce advese effects very similar to those traditionally caused by natural rain conditions.
A typical prior art control system for a permanently installed sprinkler system consists of a cycle timer in combination with a solenoid valve which consists of a solenoid coil and a metal stem. The cycle timer turns the solenoid valve on for a specified period of time, thereby permitting a sprinkler associted with the solenoid valve to disperse water supplied by a connected pipeline. To disperse water, the solenoid coil actuates the metal step which pushes or pulls on a diaphragm operating the valve. Often, the cycle timer is set well in advance of the period during which water is dispersed and the sprinkler system turned on during periods when the lawn is not in use. For example, a cycle timer may be set such that the associated sprinkler will water the lawn for one-half hour every morning. The drawback to such systems is that once set, the sprinkler system will provide the same amount of water irregardless of either the immediate weather conditions or the seasonal weather conditions. Such systems provide no control of the amount of water distributed to the ground based upon the moisture content of the ground being irrigated is provided. Thus, the control system will actuate the sprinkler to disperse water even if the ground is already sufficiently moist if the cyclee timer associated therewith is issuing the appropriate instruction. As a result, to prevent the distribution of water during a rainstorm, or to limit the amount of water dispersed to sufficiently moistened soil during a rainy season, the cycle time must be manually overridden.
There exists numerous control switches which are intended to control the operation of an electrical system associated therewith. For example, in my co-pending U.S. patent application Ser. No. 07/503,467, filed Mar. 30, 1990, I disclosed various embodiments of a two probe control switch for disconnecting an associated electrical system comprised of an AC source and a load upon the detection by the probes of an excessive liquid accumulation at a specified location. The various embodiments of the aforementioned control switch may be seen by reference to FIGS. 3b, 4a-b, 5a-b and 6a-d and the operation of each of these control switches is described in detail below. While the operation of the disclosed control switches has been generally satisfactory, accidental electrical contact between the probes and the associated electrical system may result in damage to the electrical system.
Referring momentarily to the control switch 48 illustrated in FIG. 3b, this problem will now be described in greater detail. If the first liquid detection probe 54 accidentally contacts AC line Am the rectifier diode DD2 is bypassed and a half wave or pulsing DC current can flow through the rectifier diode DD1 to the load 52. Similarly, if the first liquid detection probe 54 contacts AC line B, the rectifier diode DD1 is bypassed and pulsing DC current can flow through the rectifier DD2 to the load 52. Similar bypasses of the rectifier diodes DD1 and DD2 and resultant pulsing DC current across the load 52 can result if the second liquid detection probe 56 contacts either AC lines A or B. In the event pulsing DC current flows through the load 52, a destructive chatter can be produced in the elements of the load 52. As it may be difficult, if not impossible, to completely isolate the liquid detection probes of the control switch from the AC lines of the associated electrical system, the problems associated from such contact may be minimized by providing a ground common to the DC control switch and the AC electrical system.
Additional problems are posed by the use of a control switch to control an irrigation system based upon the level of ground moisture. As previously discussed, if the AC source voltage contacts a moisture detection probe, the solenoid which controls the flow of water to the sprinkler would begin to chatter. Electrical contact between the associated electrical system and the control switch could occur if the insulation which separates the associated electrical system from the control switch is broken. Under these circumstances, the AC source voltage could be conducted through moist ground to the control switch. In addition, during the operation of the solenoid, the solenoid stem receives a current coupling through the coil and the water transfers this voltage through the water which, inturn, transfers it to the metallic pipe which is grounded to the earth. As a result, the AC source voltage electrically connects with the control switch.